Stratified sampling is the most suitable technique for selecting customers for testing the IT support department's success in meeting service levels, as it involves dividing the population into distinct subgroups (strata) based on certain characteristics (in this case, customer size classification based on annual expenditures and service nature). This method ensures that each subgroup is adequately represented in the sample, providing more reliable and relevant results. References:
* The IIA's Global Technology Audit Guide (GTAG) on Data Analysis Technologies.
* The IIA's Practice Guide on Audit Sampling.

If an internal auditor decides to adjust the audit work program during the fieldwork phase of an assurance engagement, the most appropriate next step is to obtain approval from the engagement supervisor. This ensures that any changes to the scope or procedures are reviewed and sanctioned by the audit management, maintaining the integrity and alignment of the audit objectives.
IIA Standards: 2240 - Engagement Work Program
IIA Practice Guide: Engagement Planning

The internal auditor's objective in sorting the data by vehicle and identifying instances of refueling on the same or sequential dates is most likely to determine whether fuel purchases were legitimate and for work- related purposes. By analyzing patterns of refueling, the auditor can identify any anomalies or unusual activity that may suggest misuse or personal use of the organization's vehicles. This helps ensure that organizational resources are being used appropriately and that there are no instances of fraud or abuse.
The Institute of Internal Auditors (IIA) Standard 1220 - Due Professional Care: "Internal auditors must apply the care and skill expected of a reasonably prudent and competent internal auditor." IIA Practice Guide on "Data Analytics and Continuous Auditing"

The best possible engagement objectives are those derived from a comprehensive risk assessment that incorporates inputs from both senior management and the company's risk function experts. This approach ensures that the internal audit objectives are aligned with the organization's strategic priorities and risk landscape. By combining insights from senior management with the technical expertise of risk function experts, the internal audit activity can develop well-rounded and relevant engagement objectives that address the most significant risks facing the organization.
The Institute of Internal Auditors (IIA) Standard 2010 - Planning: "The chief audit executive must establish risk-based plans to determine the priorities of the internal audit activity, consistent with the organization's goals." IIA Practice Guide on "Internal Audit Planning"

The most appropriate approach for internal audit activity to follow up on management action plans is to create a tracking system. This ensures that follow-up activities are systematically monitored and documented. Such a system can track the status of action plans, provide reminders for due dates, and record progress updates, thus ensuring that management's corrective actions are implemented and effective. Regular monitoring and tracking are essential to verify that issues identified in audits are addressed in a timely manner.
Institute of Internal Auditors (IIA) Standards: Implementation Standards 2500 - Monitoring Progress COSO Framework: Monitoring Activities Component